The present invention relates to an f-xcex8 lens that focuses light beams on a plane and scans the light beams across the scanning plane at substantially constant speed. The present invention relates also to an beam scanning device using the f-xcex8 lens and a color imaging apparatus using the beam scanning device.
The beam scanning device is provided with a light source generating light beams, and an optical deflection device, like a polygonal mirror, that deflects the light beams in a constant direction. The deflected light beams are projected onto a plane through the f-xcex8 lens, to sweep or scan the plane in a constant direction. The f-xcex8 lens is a lens that has a property to make the light beams move on the scanning plane proportionally to a deflection angle xcex8 of the optical deflection device. In the beam scanning device, the f-xcex8 lens is used for making the scanning speed of the light beams on the scanning plane proportional to the deflection speed (the speed per deflection angle of the optical deflection device). Because the scanning plane is held flat in the deflecting or scanning direction, the scanning speed would not be proportional to the deflection speed without the f-xcex8 lens.
The f-xcex8 lens has usually been composed of two or three spherical lens elements, or one or two aspherical lens elements. In the f-xcex8 lens, the light beams pass through each spherical lenses merely in a narrow central zone extending in the deflecting direction, so other peripheral portions of the spherical lenses are cut away, to shape the spherical lenses into strips when used as the components of the f-xcex8 lens.
However, cutting the spherical lens into a strip requires is labor-consuming, because it is necessary to locate the cutting positions very strictly. In addition, since the peripheral portions of the spherical lens are thrown way uselessly, it is a waste of material, and the material cost are pretty large. It is possible to use the whole spherical lenses in the f-xcex8 lens, in order to omit the labor-consuming cutting process. But it is still a waste of lens material, as there remain unused portions in those spherical lenses. Moreover, as compared to the strip-shaped spherical lenses, the whole ones obviously require a larger space, and thus enlarges the apparatus size.
Meanwhile, in the color imaging apparatus where a plurality of light beams having different wavelengths are scanned across a photosensitive material at the same time by the beam scanning device, the scanning lengths of the light beams on the photosensitive material can differ from each other because of chromatic aberration of the f-xcex8 lens. In that case, recorded images will suffer color failures. As well-known in the art, the chromatic aberration is resulted from the fact that the refractive index through a glass or a lens varies depending upon the wavelength.
To prevent the color failure, optical correction of the chromatic aberration of the f-xcex8 lens has usually been made. For correction of the chromatic aberration, selection of glass materials for the lens elements of the f-xcex8 lens is important. Usually, lenses made of crown glass and lenses made of flint glass are usually combined in order to correct the chromatic aberration. For better correction, however, the sorts of glass materials for the f-xcex8 lens are limited to expensive ones, and also the number of lens elements must be increased. Thus, the f-xcex8 lens becomes expensive.
Indeed Japanese Laid-open Patent Application No. 6-18803 discloses an f-xcex8 lens using a cylindrical lens as one component, but the other component of this prior art is a spherical lens, so it needs a relatively high manufacture cost. Although there has been disclosed an f-xcex8 lens system whose components are all cylindrical lenses in Japanese Laid-open Patent Application No. 3-130717, the f-xcex8 lens of this prior art has a problem that sufficient lens performance cannot always be achieved because of tolerance in manufacture. To avoid this problem, this prior art suggests replacing one of the cylindrical lenses of the f-xcex8 lens with a spherical lens. However, this solution results in rising the manufacture cost.
In view of the foregoing, the present invention has an object to provide an f-xcex8 lens that has superior f-xcex8 characteristics but may be produced at a low cost, on the assumption that the f-xcex8 lens is placed between an optical deflection device that deflects light beams at an approximately constant angular speed in a deflecting direction, and a scanning plane, for converging the deflected light beams on the scanning plane such that the light beams scan the scanning plane at an approximately constant speed in a scanning direction, wherein the scanning plane is flat in the scanning direction of the light beams.
Another object of the present invention is to provide a beam scanning device and a color imaging apparatus, using the f-xcex8 lens of the present invention.
According to the present invention, the f-xcex8 lens is composed of a first lens element having a negative power of refraction, a second lens element having a positive power of refraction and a third lens element having a positive power of refraction, arranged in this order from the side of the optical deflection device, wherein at least one surface of the lens elements is a cylindrical surface that has a refractive power only in the deflecting direction.
The f-xcex8 lens of the present invention uses a cylindrical lens having a curvature only in the deflecting direction of the light beams. Such a cylindrical lens may be manufactured by forming a primary cylindrical lens whose cylindrical surface has a pretty large length in a perpendicular direction to the direction of curvature, and then cutting the primary cylindrical lens along the direction of curvature into a number of lens pieces of a constant width or height. Every one of subsequent cut pieces is usable as a component of the f-xcex8 lens, so any waste is not produced from the primary cylindrical lens. Therefore, the cylindrical lenses used in the f-xcex8 lens of the present invention may be manufactured at a low cost, so the manufacture cost of the f-xcex8 lens of the present invention is low in comparison with the conventional f-xcex8 lens using those lens elements which are manufactured by cutting round lenses into strips. It is to be noted that cutting process of the primary cylindrical lens may be done at any stage of manufacturing the individual lens elements, e.g. before finishing the surface of the primary cylindrical lens.
According to a preferred embodiment, the f-xcex8 lens satisfies the conditions: N1 greater than N2=N3, v1 less than v2=v3, and 4 mmxe2x89xa6|f23/f1xc2x7DO|xe2x89xa616 mm, wherein N1, N2 and N3 represent refractive indexes, v1, v2 and v3 Abbe represent constants of the first to third lens elements respectively, fi represents the focal length of the first lens element, f23 represents the composite focal length of the second and third lens elements, and DO a distance from the optical deflection device to the lens surface of the first lens element.
This configuration is effective to prevent curvature of the field of the f-xcex8 lens from being worsened, and maintain good speed f-xcex8 characteristics. It is to be noted that the speed f-xcex8 characteristics indicate the proportionality of the scanning speed of the light beams on the scanning plane to the angular speed of deflection. That is, with good f-xcex8 characteristics, the scanning speed of the light beams is maintained constant through the entire scanning length.
According to another preferred embodiment, at least one of the three lens elements is inclined about a parallel axis to the deflecting direction. This configuration is effective to prevent interference caused by multiple reflection of light beams between the lens surfaces. Especially by inclining the first lens element, those light beams multiple-reflected between the optical deflection device and the first lens element are prevented from being projected toward the scanning plane, so the ghosts are effectively prevented.
According to a further embodiment, the first and second lens elements are cemented to each other. This configuration reduces the risk of deviation of the lens elements from their proper positions.
A color imaging apparatus according to the present invention comprises:
a light source having a plurality of light emitting elements for emitting light beams of different wavelengths;
an optical deflection device that cyclically deflects the light beams at an approximately constant angular speed in a deflecting direction;
an f-xcex8 lens placed in optical paths of the deflected light beams from the optical deflection device, for converging the light beams on a scanning plane such that the light beams scan the scanning plane at an approximately constant speed in a scanning direction, wherein the scanning plane is flat in the scanning direction of the light beams, and at least one surface of lens elements of the f-xcex8 lens is a cylindrical surface that has a refractive power only in the deflecting direction;
a clock signal generating device that generates a plurality of clock signals of different frequencies for deciding output timings of the light beams such that the light beams scan the scanning plane from an equal scanning start point through an approximately constant length in the scanning direction; and
a light modulation device for modulating the light beams on the basis of the clock signals and image data.
The color imaging apparatus of the present invention shifts the output timings of the light beams of different wavelengths so that the scanning start position of these light beams on the scanning plane will coincide with each other. That is, chromatic aberrations of the f-xcex8 lens are compensated a certain degree by shifting the output timings of the light beams. Therefore, the f-xcex8 lens is not required to correct the chromatic aberrations entirely by itself. So the f-xcex8 lens may be manufactured at a low cost, using inexpensive lens elements that may be produced at a high efficiency from inexpensive glass materials, without lowering the lens performance or the image quality.